1. Field of the Invention
The invention relates generally to vehicle coolers and more particularly to a device for cooling the interior passenger space of a vehicle such as truck or recreational vehicle.
2. Description of the Related Art
It is customary for vehicle manufacturers to include air conditioning systems in their vehicles to provide cooling air to the vehicle interior. Such systems are now standard on many models and generally work in a similar way to domestic air conditioners. The air to be cooled is passed over a heat exchange element. Within the heat exchange element, a refrigerant absorbs heat from the air and transports it away. The refrigerant is driven around a closed circuit by a compressor. Known air conditioning devices of this type require substantial energy to drive the compressor and additionally often make considerable noise.
Energy consumption is increasingly of significance to consumers. For users of vehicles with such air conditioning systems, this may manifest itself in the form of increased fuel consumption. Additionally, such air conditioning systems may require more power than a battery supply can provide for more than a short period. To maintain operation of an air conditioner in a stationary vehicle the engine must be left running to prevent the battery from discharging. For trucks and buses running on idle while the driver rests, the noise, fumes and energy consumption are all a significant problem.
In an alternative frequently used by recreational vehicles and caravan trailers, an additional generator may be used to provide power supply to operate the air conditioner and other appliances. Such generators may be located at a distance from the vehicle but also causes additional noise to that already produced by the air conditioning compressor. They also lead to additional energy consumption and are not suitable for being left on for long periods.
There is thus a particular need for a vehicle cooler that can operate quietly with minimal power supply. Ideally, the cooler should be able to provide adequate cooling to a vehicle interior for an extended period using only an existing vehicle battery supply.
One type of cooling device that operates without a compressor is the evaporative cooler. The principle of evaporative cooling has been known for many centuries. For example, a damp cloth placed over an object will keep the object cool by evaporation of liquid from the cloth. By continuously adding liquid to the cloth, the cooling effect may be maintained indefinitely without input of electrical energy. An evaporative cooler makes use of this principle. An air stream over a first wetted surface of a heat exchange element absorbs moisture from the surface by evaporation. A second stream over a second surface of the heat exchange element may then be cooled by contact with the second surface. The lowest temperature that can be reached by evaporation of moisture in this way into an air stream defines the wet-bulb temperature for that air.
If a quantity of air is cooled by direct evaporation its absolute humidity increases due to the uptake of moisture. Its relative humidity also increases due to its lowered temperature until at the wet bulb temperature it is full saturated with water vapour. If the air is cooled however without direct evaporation, its absolute humidity remains the same. As its temperature decreases only the relative humidity increases until full saturation of the air is reached at the so-called dew point. The dew point is thus lower than the wet bulb temperature and is in fact defined as the temperature to which a body of air must be cooled to reach saturation or 100% relative humidity. At this point, water vapour in the air condenses.
Evaporative coolers have however been found relatively inefficient in lowering the temperature of the ambient air to acceptable values. In order to provide sufficient cooling, especially in humid climates, a large heat exchange surface is required. Evaporation of the water is often not optimized and only a part of the available energy that could be used for cooling is actually used. As a result, water consumption is high. Such devices have also been found impractical for use on most vehicles due to their relatively bulkiness for a given cooling capacity.
One device which attempts to overcome the problems of evaporative cooling for use in vehicles is known from U.S. Pat. No. 6,497,107 to Maisotsenko et al. In this document, use is made of the evaporation of the fuel for an internal-combustion engine to provide cooling. The product fluid in such cases must be isolated from the working fluid to avoid any chance of the evaporated fuel entering the passenger space of the vehicle. Furthermore, the amount of fuel available for evaporation is limited and the system is not effective once the engine has stopped. An alternative device for use in vehicles without fuel is also described. This device needs to be combined with a desiccant or other device for drying the air prior to moisturizing. The requirement of such desiccant or additional heating means makes the device unsuitable for continued operation on a low power supply such as is available from a normal vehicle battery.